Light is a form of electro mechanical radiant energy that is detected visually or by appropriate sensors. Light is considered to have a cyclic wave energy pattern and ordinarily move or project in a straight line called a ray. When light energy as an incident ray strikes an object some is reflected, some is absorbed and some is transmitted. Conventionally light travels in a straight line and may be reflected or subject to refraction in the same manner.
Refraction is the recognized scientific principal which causes a bending or change in the direction of the ray or propagation of light as it passes or is transmitted from one light transparent material or medium to another. Such refraction bending is considered completely reversible in as the identical bending occurs in either direction. Both the angles of incidence and refraction (or reflection) are conventionally determined by reference to the normal which is located at right angles to the plane of the interface of the two mediums or materials. In reflection, the angle of light ray incidence is equal to the angle of reflection.
The angular path of the refracted ray is determined by the index of refraction of the material receiving the incidence light ray. The absolute index of refraction of any medium is defined as the index of refraction of the medium relative to vacuum. The absolute index of refraction is usually called just the index of refraction on the refractive index of the medium. The greater the index of refraction the greater the optical density and the greater the angle of bending.
Total reflection is another light energy phenomenon that occurs at the interface of two mediums of different optical density (an optically denser material inherently also has a higher index of refraction). A light incident ray within the denser medium having an incidence angle to give an angle of refraction greater than 90.degree. is totally internally reflected and no light passes into the other medium. The angle of incidence for which the angle of refraction is 90.degree. or normal is called the critical angle. Light at angles of incidence at or greater than the critical angle is totally internally reflected within the denser medium. Total internal reflection can only occur only for light within a medium of higher optical density at an interface surface with a medium of lower density. Although the principle is based on the second law of refraction, the effect is called total internal reflection.
Fiber optics employ the total internal reflection to achieve transmission of light with great effectiveness and relatively small light loss or attenuation. Fiber optics serve as dielectric optical waveguides or conductors for directing the propagation of light in a selected path. Because fiber optics are basically passive devices (no moving parts) they are durable and simple in operation and therefor highly desirable from a reliability and maintenance standpoint.
Fiber optic sensors using the principle of internal refraction are known. For examples see U.S. Pat. Nos. 3,282,149, 4,286,468 and 4,564,292. All of these determine the amount of light transmitted through a single optic waveguide fiber as a function of the light loss occurring at a selected sensor portion.
U.S. Pat. No. 3,282,149 to Shaw et al. is entitled "Linear Photoelectric Refractometer". The Shaw patent sets forth the then state of the art in some detail including certain mathematical light relationships. In addition, by measuring the transmitted light through a straight transparent rod where the index of refraction of the surrounding material is less than that of the rod, some of the light reflected at angles less than the critical incidence angle and all of the light reflected at angles greater the critical angle will be retained (total reflection) in the rod. By measuring the transmitted light in the rod, the index of refraction of the surrounding material can be mathematically determined. The actual source of light is not critical and good results are obtained from an ordinary heated tungsten illumination element light bulb.
A conventional photo detector system is used to measure the value of the light retained in the rod and generate a suitable responsive electrical signal. Using known apparatus, this electrical signal is processed in a predetermined manner and the desired information is then usefully displayed. The disclosed device of the Shaw patent is used for measuring the index of refraction of a fluid using a helically coil shaped transparent sensing body having at least 360.degree. of curvature and made of a transparent dielectric (electrically insulating) material having an index of refraction higher than that of the substance to be measured. The sensing element shape enables the determination of the index of refraction without the necessity to determine the angle of incidence optically. Such arrangement is particularly desirable if a photoelectric sensor or detector is used to measure the light intensity for making the determination. Because the light intensity in the rod is measured, the presence of color, bubbles or solids in the fluid being measured for refraction does not adversely affect the information generated.
U.S. Pat. No. 4,564,292 to Omet also discloses a portable photoelectric refractometer for measuring the index of refraction of a sample medium. The disclosed device measures the transmission of light through a U-shaped or curved unshielded fiber optic sensor mounted on a probe. As the light transmission is a substantially linear function of the index of refraction greater accuracy and reliability can be obtained in use if the temperature factor can be controlled. To eliminate the potential source of temperature error, a reference U-shaped fiber optic sensor is placed in the probe in contact with a reference fluid along with a similar shaped fiber optic sensor for contacting the fluid being examined. By a suitable delay in making a reading until a common temperature is reached, the potential for measurement error from a temperature difference of the fluids are eliminated.
In U.S. Pat. No. 4,286,468 to Altmen a method and apparatus for sensing or transducing sound wave motion by determination of total light transmitted by an optical fiber is disclosed. This patent, which is entitled "Frustrated Total Internal Reflection Fiber-Optic Small Motion Sensor For Hydrophone Use," discloses a hydrophone transducer for detecting acoustic pressure wave signals in the ocean. The hydrophone transducer operates by sensing a change in the refractive index of the fiber as the acoustical pressure changes. Such changes in refractive index effect a phase delay of the transmitted light which is then measured and compared with a reference to detect the sound waves.
The unclad spiral portion of the Altmet Patent fiber optic sensor is positioned adjacent to a flat plate and the space therebetween filled with a fluid having a low optical loss characteristic and a refractive index lower than the fiber core. Because of the lower refractive index, the transmission of light through the unclad coiled fiber optic sensor is dependent on total internal reflection at the core interface. Also present adjacent the unclad sensor portion is an evanescent light wave field which externally surrounds the sensor. The evanescent light field is made available or created by removing the optical cladding. To the extent that this surrounding light field is intercepted by a material of higher refractive index than the fluid, the total internal reflection is diminished and the apparent light transmission loss of the optic sensor increased. The closer the movable field light interceptor plate moves to the fiber optic sensor the greater the apparent loss of light. Because the plate frustrates the otherwise total internal reflection of the fiber optic sensor, this type of modulation phenomenon is sometimes called "frustrated total internal reflection". The resulting change in the phase, not intensity, of the fiber optic transmitted light can be measured and displayed using conventional techniques.
Another group of fiber optic devices uses a liquid core having a quartz cladding. By modifying the refractive index of the liquid core to vary the amount of light entering into or escaping from the core a useful feature is obtained. Examples of such devices are disclosed in U.S. Pat. Nos. 3,819,250 and 4,201,446.
Kibler U.S. Pat. No. 3,819,250 is entitled "Temperature Sensitive Fiber-Optic Devices" and discloses three embodiments of a low light loss fiber optic coupler invention. The embodiment of FIG. 1 is a wide aperture coupler while the embodiment of FIG. 3 is a directional light coupler. FIG. 2 is a small radius bend guide coupler used to reduce light loss. All three embodiments utilize a temperature control to enhance effectiveness of the device. The reversible wide opening or aperture coupling enables the quartz cladding to introduce light into the liquid core at a large critical angle in the heated region. The light accepting property of an optical fiber is normally called the numerical aperture and often limits the amount of transmitted light captured. The numerical aperture, a mathematical measure of the light-accepting property of the sensing fiber optic core is controlled by the temperature of the coupling fluid. In the embodiment of FIG. 1 the disclosed fiber optic sensors employ a quartz cladding or coating and a dielectric liquids to enhance the light accepting or radiating coupling.
The normal light loss in the bend coupler (FIG. 2) is reduced by cooling the core and cladding to increase the difference in refractive index and increase the total internal reflection.
The directional coupler embodiment (FIG. 3) employs two parallel quartz fiber optic rods embedded in a dielectric coupler liquid (carbon tetrachloride). While structurally similar to the present invention, the disclosed function, operation and purpose are entirely different. Controlled temperature variations of the disclosed coupler structure will transmit or couple different amounts of refracted light between the two rods due to the relative change in the indices of refraction of the liquid and the quartz rods. With this arrangement the amount of light coupled from one rod to the other is determined by the external temperature controller.
U.S. Pat. No. 4,201,446 to Geddes et al. discloses a fiber optic sensor device for determining an unknown temperature. The disclosed sensor is a liquid-core optical fiber contained in a transparent glass capillary tube that is mounted in a conventional single fiber optic conductor at a desired remote sensing location. The liquid core of the sensor portion has a temperature dependent index of refraction over a given temperature range. The numerical light aperture of the disclosed photoelectric electric sensor varies continuously from zero to the maximum value sensed by the fiber optic sensor. In another embodiment a conventional unclad fiber optic core is immersed in a liquid having a temperature sensitive refractive index.
Fiber optic core interconnection is known and essential for commercial operation whether classified as a connector, a splice or a coupler. Splices are usually considered to be fusion or other permanent end-to-end joints between the cores of two separate fibers. Connectors are usually considered to be demountable interconnections while couplers are usually considered optical connectors that redistribute particular light energy between two or more fibers. Collectively all of these devices seek to connect separate fiber optic fibers with a minimum of attenuation or loss of light.
Also, fiber optic couplers have been devised using a liquid medium to couple light from one fiber to another either from butt end to butt end or from unclad core to unclad core via the cylindrical surfaces. See, for example, previously mentioned U.S. Pat. No. 3,819,250 (FIG. 3 and description at col. 4 lines 40-56) regarding an external temperature controlled directional light coupler. As noted previously the coupling relies on external temperature control of the fluid to a change the refractive index to permit light transmission.
The basic transduction mechanism employed in many known fiber optic sensors is the phase modulation of coherent light guided through a section of single mode fiber by the action of a detected energy field. Chapter 4 of the publication entitled "Fiber Optic Sensor TECHNOLOGY HANDBOOK" published by Dynamics System, Inc. describes the known forms or configurations currently employed in fiber optic sensors. A common aspect of the mentioned transducers or sensors is the splitting and recombing of the light beams to determine a light phase shift for measuring the condition with such transducers.